Resins for extrusion coating on paper, board, aluminum, etc., are designed with broad molecular weight distribution and low extractables. In extrusion coating applications, the polymer is processed at high temperature conditions, typically above 280° C. and below 350° C. Broad molecular weight distribution (MWD) resins with a very high molecular weight fraction are used for good processability during coating (neck-in and drawdown balance). Low extractables are needed to reduce undesirable taste and odor, and to reduce smoke formation during the processing of the resin, especially during the high coating temperatures. Typically LDPE (low density polyethylene) resins with broad MWD are made using autoclave reactors or a combination of autoclave and tube reactors. Broad MWD resins can be made by promoting long chain branching, and through the inherent residence time distribution, by which molecules will undergo shorter (low molecular weight) or longer (high molecular weight) growth paths. Broad MWD autoclave resins for LDPE extrusion coatings are focused in two product density regimes, namely from 0.915 to 0.919 g/cc and from 0.919 to 0.924 g/cc. This invention describes improved broad MWD tubular reactor products designed for the higher density regime from 0.919 to 0.924 g/cc. The autoclave and tubular reactor systems differ in residence time distribution, which is more uniform for tubular reactors and dispersed for autoclave reactor zones.
The uniform residence time leads to narrower MWD, and very broad MWD can only be achieved in tubular reactors by applying extremely differentiated polymerization conditions, for example, as described in WO 2013/078018, and/or application of a branching/cross-linking agent, for example, as described in U.S. Pat. No. 7,820,776. The use of extreme process conditions and/or costly branching/cross-linking agents can lead to high melt strength tubular low density polyethylene, suitable for extrusion coating applications; however with elevated extractables. Undesirable gels in the polymer can result from the use of branching or cross-linking agents. Due to the difference in cooling capability, the conversion level ranges, typically, from less than 20% (autoclave) to more than 30% (tubular). This large difference in conversion level has a major impact on investment and operation costs, as well on polymer output and power consumption (to compress ethylene) per unit of polymer. U.S. Publication No. 2008/0242809 discloses a process for preparing an ethylene copolymer, where the polymerization takes place in a tubular reactor, at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (meth)acrylate. Other polymers and processes are disclosed in EP 2 681 250 A1, WO 2007/110127; WO 2014/003837; WO2013/078018; WO2013/078224; WO2013/178241; WO2013/178242; WO2013/149698; WO2013/132011 and WO2013/083285. There is a need for ethylene homopolymers made at density ≥0.919 g/cc, with broad MWD, high G′ value and at reduced extractable levels, in a tubular reactor, at high ethylene conversion levels. Further, such ethylene homopolymers should be cost effective to make, not requiring additional costs associated with modifying or branching agents. These needs have been met by the following invention.